In a mobile wireless communication system, in general, a radio wave from a base station reaches a mobile terminal apparatus (to be referred to as a terminal apparatus hereinafter) such as a mobile telephone through a plurality of paths. More specifically, a plurality of incoming waves are received by the terminal apparatus while being added to each other. At this time, by interference (multipath interference) of the plurality of incoming waves, a reception electric field intensity in the terminal apparatus varies with elapsed time. Furthermore, when the terminal apparatus moves, multipath aging in which the reception electric field intensity varies rapidly and intricately becomes conspicuous with the movement. A digital signal transmission characteristic is deteriorated due to the multipath aging.
In 3GPP (3rd Generation Partnership Project) which is an international standard specification association of a third generation mobile wireless communication method, a new communication technique called an HSDPA (High Speed Downlink Packet Access) which can perform high-speed wireless communication is added. When the HSDPA is used, a data transfer speed which is three or more times as fast as that obtained by using W-CDMA (Wideband Code Division Multiple Access) can be realized. However, when the HSDPA is operated in an actual environment, a high data transfer speed cannot be easily achieved by multipath interference.
Therefore, in order to compensate for signal deterioration in a transmission path to reproduce signal data, a circuit to remove influence of multipath interference must be mounted on a terminal apparatus. As such an example of the circuit, an equalizer is given (for example, see Patent Document 1).
FIG. 1 is a block diagram showing a configuration of the equalizer described in Patent Document 1. The equalizer shown in FIG. 1 is an equalizer using an NLMS (Normalized Least Mean Square) algorithm. In the equalizer shown in FIG. 1, an input signal x(n) is input to a filter coefficient computing unit 51 and an equalization filter 52. The input signal x(n) is, for example, a signal obtained by A-D converting a signal received through a wireless transmission path. An output from a difference detecting circuit 53 is also input to the filter coefficient computing unit 51. The difference detecting circuit 53 outputs a difference signal e(n) between an output signal y(n) from the equalization filter 52 and a spread code of a pilot channel (pilot signal spread code) d(n).
The filter coefficient computing unit 51 uses a filter coefficient (tap coefficient) w(n) before updating and the difference signal e (n) to perform an arithmetic operation expressed by the following equation so as to generate a new filter coefficient w(n+1). The filter coefficient w(n+1) is output to the equalization filter 52. Reference symbol “*” denotes multiplication.w(n+1)=w(n)+μe(n)*x(n)
where μ is a step size parameter, and is expressed by the following equation.μ=α/(x(n)H*x(n)+β)whereβ: stabilization parameter (parameter to preventing the denominator from being 0, and sufficiently small positive value)α: parameter to determine a convergence property of NLMS algorithmx(n)H: transpose conjugate vector of x(n)
The equalization filter 52 has a part to which (f−1) delay units are continuously connected. The input signal x(n) is sequentially delayed by the (f−1) delay units. Furthermore, an input signal input to an input terminal (input terminal for an input signal) and an output (total of f data) from the (f−1) delay units are multiplied by a corresponding filter coefficient of f filter coefficients output from the filter coefficient computing unit 51. And f multiplication results are summed up to obtain an output signal y(n).
The equalization described in Patent Document 1 is constituted as described above, and operates as described above. In this manner, influence of multipath interference can be effectively removed. Patent Document 1 discloses that a despreading process for channel estimation is performed in the equalizer.
As a signal receiving apparatus in a mobile wireless communication system using a spread spectrum technique, a signal receiving apparatus in which a despreading circuit for performing a despreading process is connected to an output terminal of an equalizer and a signal receiving apparatus in which an equalizer is connected to an output of a despreading circuit are known (for example, see Patent Document 2).
Patent Document 1: JP-A 2005-175775 (Japanese Patent Laying-Open No. 2005-175775) (paragraph 0004, 0023, FIG. 1, FIG. 5)
Patent Document 2: JP-A 2004-40305 (Japanese Patent Laying-Open No. 2004-40305) (paragraph 0009, 0032, 0035, FIG. 10, FIG. 11)